A class of parallel concatenated convolutional codes, also known as PCCCs or turbo codes, is known for example from an article by C. Berrou et al. entitled "Near Shannon Limit Error-Correcting Coding And Decoding: Turbo-Codes", Proceedings of the IEEE International Conference on Communications, 1993, pages 1064-1070. That article showed that a turbo code together with an iterative decoding algorithm could provide performance in terms of BER (Bit Error Rate) that is close to the theoretical limit. A turbo code encoder provides a parallel concatenation of two (or more) RSC (Recursive Systematic Convolutional) codes which are typically, but not necessarily, identical, applied to an input bit sequence and an interleaved version of this input bit sequence. The output of the encoder comprises systematic bits (the input bit sequence itself) and parity bits which can be "punctured " (selected) to provide a desired rate of encoding.
Various schemes are being proposed and developed to provide, especially for the communication of data in a CDMA (code division multiple access) communications system a greater bandwidth (signal transmission rate) than is provided in a so-called IS-95 system which is compatible with TIA/EIA (Telecommunications Industry Association/Electronic Industries Association) Interim Standard IS-95-A, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System". Turbo coding has been proposed for such WCDMA (wideband CDMA) systems. However, turbo coding does not provide a great increase in code distance, which is a significant disadvantage for a BER of less than about 10.sup.-5 which is desirable for WCDMA systems.
It is desirable to optimize the application of turbo coding to WCDMA systems, in order to obtain maximum coding gains.
An object of this invention is to provide improved encoding and decoding methods and apparatus.